A 72-year-old man with general weakness visited the outpatient clinic of the hematology department. The patient had been treated under the diagnosis of autoimmune hemolytic anemia for 2 years. His hemoglobin level at the time of the visit was 6.3 g/dL, and a blood transfusion was requested to treat his anemia. The patient’s blood type was A, RhD positive. Antibody screening and identification test showed agglutination in all reagent cells with a positive reaction to autologous red blood cells (RBCs). He had a prior transfusion history with three least incompatible RBCs. The patient returned home after receiving one unit of leukoreduced filtered RBC, which was the least incompatible blood in the crossmatching test. After approximately five hours, however, fever, chills, dyspnea, abdominal pain, and hematuria appeared and the patient returned to the emergency room next day after the transfusion. The anti-Fya antibody, which was masked by the autoantibody, was identified after autoadsorption using polyethylene glycol. He was diagnosed with an acute hemolytic transfusion reaction due to anti-Fya that had not been detected before the transfusion. In this setting, it is necessary to consider the identification of coexisting alloantibodies in patients with autoantibodies and to become more familiar with the method of autoantibody adsorption.